The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include gerotor displacement mechanisms utilizing low-speed, commutating valving.
In conventional gerotor motors utilizing low-speed, commutating valving (i.e., the rotary valve element rotates at the speed of rotation of the gerotor star rather than the orbiting speed of the star) the valving action has been accomplished by means of a rotary valve member and a stationary valve member, with both valve members being separate and distinct from the gerotor displacement mechanism. One disadvantage of the conventional gerotor motor valving arrangements has been the occurrence of "timing" errors, especially in motor designs in which the rotary valve element was driven by the motor output shaft or the dogbone shaft. When torque wind-up of the dogbone shaft occurs, the relative position of the gerotor star and the rotary valve deviates from the theoretical position, resulting in an error in the valve "timing", i.e., the communication of fluid into and out of the volume chambers as they expand and contract. Another disadvantage of arrangements in which the stationary and rotary valve elements are separate from the gerotor mechanism is simply the excessive number of parts required and the resulting expense.
It has been recognized for a number of years that one solution to the types of problems mentioned above is the provision of a gerotor motor in which a portion of the gerotor star itself comprises the rotary valve member ("valve-in-star"). It has been recognized that a valve-in-star design should substantially eliminate valve timing errors because of the fixed relationship between the star and the rotary valve ports. In addition, having fewer elements surrounded by leakage clearances and fewer elements requiring some sort of pressure balancing results in a motor capable of achieving both higher volumetric efficiency as well as higher mechanical efficiency. U.S. Pat. No. 3,825,376 illustrates one fairly early attempt at a valve-in-star design. However, each of the rotary ports associated with the gerotor star opened directly into the volume chamber, thus interrupting the star profile, which has long been recognized as being undesirable. In addition, the device of U.S. Pat. No. 3,825,376 shows each of the rotary star ports being disposed in the star valley which means in a motor having five volume chambers, there are at least periodically times when three pockets are in a changeover condition, while only one pocket is communicating with the pressure inlet and only one pocket is communicating with the exhaust port. The result of such an arrangement will be excessive variation in motor output torque ("torque ripple"), as well as an undesirable frequency of "trapping" of fluid within the volume chambers which are momentarily not in fluid communication with either the inlet port or the outlet port.
A more recent attempt to provide a satisfactory valve-in-star gerotor motor is illustrated in U.S. Pat. No. 4,411,606, in which the "manifold valving" or directional valving occurs between the star and the endcap, while the commutating valving occurs at the axially opposite end face of the star, at the interface of the star and an adjacent valve plate. Such an arrangement effectively requires that the valving be "fixed clearance", as opposed to being pressure balanced or pressure overbalanced.
In addition, the arrangement in U.S. Pat. No. 4,411,606 requires a plurality of axial bores extending through the star to communicate between the opposite ends of the star. If such bores are fairly small, there is too much flow restriction, and too large a pressure drop within the motor, which reduces mechanical efficiency of the motor. On the other hand, if such bores are large enough to avoid excessive flow restriction, the result is a weakening of the star.
Accordingly, it is an object of the present invention to provide an improved low-speed, high-torque gerotor motor utilizing a valve-in-star design which substantially overcomes the problems of the prior art devices.
It is a further object of the present invention to provide a device in which both the manifold valving action and the commutating valving action occur at the interface of the gerotor star and the endcap disposed adjacent the star.
It is another object of the present invention to provide a device which accomplishes the above-stated objects without the need for a separate plate member disposed between the gerotor gear set and the adjacent endcap.
Low-speed, high-torque gerotor motors of the type to which this invention relates have typically been utilized in systems in which the relief valve would be set at approximately 3,500 psi, and in which the motor would operate at approximately 3,000 psi. More recently, there has been increasing demand in the marketplace for motors capable of operating at relatively higher pressures, at least intermittently, in systems in which the relief valve may be set as high as 4,500 psi or even 5,000 psi.
In the valve-in-star motor shown in above-cited U.S. Pat. No. 3,825,376, the variation in the number of volume chambers communicating with the ports, and the resulting torque ripple, make the motor shown therein unsuitable for high-pressure applications.
The motor shown in above-cited U.S. Pat. No. 4,411,606 is similarly unsuitable for high-pressure applications because of the "fixed clearance" type of valving which is inherent by virtue of valve action occurring between opposite end faces of the star and adjacent members fixed to the end surfaces of the gerotor ring. As is well known to those skilled in the art, subjecting a fixed clearance valve to relatively higher pressures would result in excessive "cross-port" leakage, and reduced volumetric efficiency.
Accordingly, it is another important object of the present invention to provide an improved low-speed, high-torque gerotor motor utilizing a valve-in-star design wherein the motor is capable of being used in relatively higher pressure applications.